A new model is proposed for production of fine particles of Nabutemon by rapid expansion of supercritical solutions (RESS). A mathematical model is used for prediction of particle size and particle size distribution of RESS produced particles. In this model the effect of various operating parameters such as pressure and temperature of extraction and temperature and pressure of expansion on the characteristics of products were studied. The calculation results showed good agreement between the calculation results and the experimental data for Nabutemon particles.